332 DR GEORGE WILSON ON THE 



readily examined, and I see no reason to doubt that photographic images of 

 the retina and choroid may be obtained and preserved on collodionised or other 

 actino-sensitive surfaces. 



By means of such Ophthalmoscopes, in the hands of Helmholtz, Coccius, 

 Rhete, and others, it has been placed beyond question, that much of the light 

 which enters the human eye is reflected from the anterior surface, or from some 

 depth within the layers, of the retina, without entirely traversing that membrane, 

 or reaching the choroid, so as to be subjected to the absorptive action of its dark 

 pigment. So far, therefore, as the accepted theory of vision demands that the 

 light which has reached the retina shall not return across the chamber of the 

 eye, it must be abandoned ; and the evils which are supposed to be inseparable 

 from such cross lights, or luminous reverberations, are encountered at every mo- 

 ment by every eye, whether animal or human, at least in the case of the Verte- 

 brata, which is engaged in the exercise of vision ; yet it is vision so marred and 

 obstructed which we are in the habit of calling perfect. We must plainly mend 

 our theory, or our language, for they are inconsistent with each other. 



Before seeking to determine which must be altered, it is important to notice, 

 that, according to the views of certain recent writers on the eye, among whom I may 

 specially name Kolliker and Henry Muller, the German physiologists, it is not 

 the anterior part or face of the retina, but its deeper portions which are optically 

 sensific ; and light must penetrate to them before the sensation of vision can be ex- 

 perienced. If this view be well founded, then we may be led to the somewhat start- 

 ling conclusion, that much of the light which is reflected from the retina, at the best 

 contributes nothing to vision whilst within the eye, even if we deny that it posi- 

 tively obstructs it. And if we suppose, with the older authorities, that all the light 

 which reaches the retina penetrates it sufficiently deeply to excite luminous sen- 

 sation, then it is manifest that so far as that portion which is reflected outwards 

 is concerned, the dark surface of the choroid is quite superfluous.* 



* 1 am indebted to my pupil, Mr James Wardrop, an accomplished theologian and naturalist, 

 for an abstract of the views of Kolliker and H. Muller. Their opinions ai*e contained in Kolliker' s 

 Micr. Anatomie, B. 2, Zweite Halfte 606-720. See also H. Muller's Remarks on the Structure 

 and Function of the Retina, translated in the Quarterly Journal of Micr. Science, July 1853, pp. 

 269-273. Without entering into the minute discussion of anatomical questions which I am not 

 competent to decide, it may be noticed, that both the skilful observers referred to, agree in denying 

 to the fibres or expansion of the optic nerve, the function of perceiving " objective light." This func- 

 tion belongs, according to them, to the deepest of the five layers of which they regard the retina of 

 vertebrate animals as composed. This layer, which is immediately in front of the choroid, is thus 

 described by Kolliker : — " The external or bacillar layer consists of the • rods and bulbs' (bacilli 

 et coni), whose ends evenly terminated, form a sort of mosaic pavement towards the black pigment, 

 and which internally are continued by fibres (Muller's fibres) through the three succeeding retinal 

 layers, to abut abruptly, and with radiating terminations on the external aspect of the fifth layer, 

 which, however, they do not penetrate." This fifth layer is a very delicate membrane, investing the 

 entire internal or anterior aspect of the retina. 



Accepting this description as well-founded (and it has received the approbation of the majority of 

 recent physiologists), it appears that light must more or less traverse the anterior layers of the retina, 

 till it reaches the " rods and bulbs," with their connecting radiating fibres, before it can excite a 



